A number of applications exist wherein it is desirable to selectively apply heat to a moving web, which is subject to drying by other means, for the purpose of eliminating wet streaks or areas of higher moisture concentration. This process of selectively applying varying amounts of heat across a moving web for the purposes of eliminating moisture variation across the web will hereinafter be termed "profiling". For practical reasons, the energy density must be high to achieve profiling in drying operations. Therefore, fossil fuel burners or emitters are preferred rather than electric energy. The problem then becomes one of controlling the amount of fuel or the amount of combustible gases delivered to individual burners or emitters in such a manner as to effect profile control in increments corresponding to the moisture variations across the web without turning off the burner or emitter.
For example, in the paper making field, paper is produced in the form of an elongated web, which web is comprised of wood pulp saturated with water. The water is removed from the wood pulp by squeezing the wood pulp as it passes between cooperating rollers and further by drying the web formed by the wood pulp through suitable drying means in order to reduce the moisture content to a value with a controlled range. An instrument for detecting moisture content is typically utilized to monitor moisture content of the moving web. The instrument may be located either upstream relative to and/or following the location of the dryer units. The variation in moisture content across the width of the moving web, i.e. in a direction transverse to the direction of movement of the web, frequently presents a serious problem for effectively and efficiently drying the web. To maintain a given moisture range in the final product, the moving web often has to be remoistened and/or overdried, resulting in expensive waste of energy, reduction in machine productivity, increased manufacturing cost and sacrifice of product quality. It is thus extremely desirable to provide apparatus for controlling the web drying operation in a localized manner to obtain the desired moisture range while, at the same time, either eliminating or significantly reducing the above mentioned disadvantages.